In a vacuum processing chamber ion sources are used to change the properties of substrate surfaces. Gas is fed through an electric field in a vacuum chamber to excite the gas to a plasma state. The energized ions of gas constituents bombard the surface of the substrate. The effect that the ions have on the surface is dependent on their atomic constituents and their energy.
Low-energy, ultra-clean flows of plasmas are required, to obtain a crystalline film with high crystal quality as required in the case of gallium nitride (GaN). To avoid ion damage the ion energy must not be too high, for instance, an ion with a kinetic energy of 20 or more eV for GaN must be considered a high energy ion. When an ion hits the surface during crystalline film growth and its kinetic energy is high enough to displace atoms which are already in place, a defect is created. So the materials grown with energetic plasma streams tend to have lots of defects. The defects create "carrier" densities for semiconductors such as GaN, leading to the creation of a material with n-type doped properties. In these cases it is difficult to obtain a p-type doped material. To overcome this problem a source of low energy ions is needed.
A nitrogen plasma flow or low-energy ion beam (ion energy of order 30-50 eV) is usually obtained by using a Kaufman ion source or an Electron-Cyclotron-Resonance (ECR) plasma source.
The drawback of a Kaufman source is that a hot tungsten filament is used as a cathode. The tungsten filament delivers large quantities of electrons by thermionic emission, which sustain the low-energy non-self-sustained arc discharge, but tungsten atoms evaporate from the filament and can be found in the stream of plasma and the growing films. This is not acceptable for instance in the growth of GaN because the stream is not clean and the film properties are altered by the impurities.
An ECR plasma source necessarily operates with a high magnetic field to fulfill the resonance condition of microwave frequency and electron cyclotron frequency. Typically, the standard microwave frequency of 2.45 GHz is used, leading to a required magnetic field of 875 Gauss. The gaseous microwave plasma is produced in the region of the resonance magnetic field. The ions gain kinetic energy when leaving the location of the high magnetic field and streaming towards the substrate. When a plasma is made this way there is a significant energetic component in the ion energy distribution, i.e., ions having 30-50 eV of kinetic energy are abundant. This energy is too high for the growth of a high quality crystalline films. Although ECR plasma sources are cleaner than Kaufman sources, ion damage is observed in growing films due to the relatively high ion energy. One way to overcome this energy problem is to bias the substrate electrically, to deflect the energetic ions but in doing so the low energy ions are also deflected and the growth rate decreases.
A better (cleaner) source of low-energy gaseous ions is needed to deposit high quality thin films on substrates in both research and commercial applications. This need includes not only MBE-type but also IBAD-type deposition of thin films (MBE=molecular beam epitaxy; that is film growth with reactive, activated gases; IBAD=ion beam assisted deposition, that is film growth assisted by the moderate kinetic energy of ions such as argon).
A plasma discharge chamber usable for an ion beam source, electron beam source, and a spectral light source was introduced by V. I. Miljevic and is described in several papers (Rev. Sci. Instrum. 55 (1984) 931; Rev. Sci. Instrum 63 (1992) 2619) (also see U.S. Pat. Nos. 4,871,918; 4,906,890). A preliminary explanation of the working principle of the discharge is given in a paper published in Plasma Sources Science & Technology, Vol. 4. (1995) p.571.
In one configuration as shown in FIG. 1, a gas flows through a discharge chamber 20 which consists of a metal cathode 22 (grounded) and a metal anode 24 (positively biased) separated by a TEFLON (e.g., PTFE) insulator 26. A flange 28 holds the anode 24 in place and seals it against the cathode flange using a series of O-rings 30. By applying a sufficiently high voltage (500 V or more) to the electrodes, a glow discharge ignites in the flowing gas. The gas is introduced through an opening 40 in the cathode, and leaves the source through a small aperture 38 in the anode. A high positive voltage is applied to an extraction electrode 32 leading to acceleration of the ions from the source 20 in the direction shown by arrow 36. A high negative voltage would accelerate electrons, turning the source into an electron beam source. An electromagnetic coil 34 produces a magnetic field around the anode 24 focusing the ions in an ion beam (whose direction is shown by the arrow 36) departing from the discharge opening 38 in the anode 24. Gas pressure supplied to the gas inlet 40 provides the motive force to discharge the ions from the discharge chamber 20. The extraction electrode 32 and magnetic coil 34 assist in accelerating and focusing the ion discharge into a beam. The anode 24 is insulated from the grounded cathode 22 and grounded support flange 28 by a thin film of ceramic coating deposited on the respective mating surfaces of the anode 24.
The feature which distinguishes this kind of discharge from an ordinary glow discharge is the actual exposure of a very small area of a large cross section anode facing the cathode, to the gas. In the Miljevic configuration this effect is obtained by blocking nearly all of the anode 24 by using an insulator 26, except for a small discharge aperture. This discharge aperture forms a small hollow anode, and Miljevic named the discharge "hollow-anode discharge". We have found (Plasma Sources Science & Technology, Vol. 4. (1995) p.571.) that a voltage drop appears in front of the discharge opening, accelerating electrons which gain enough energy to ionize the working gas through inelastic collisions. A bright "anode plasma" forms in the anode channel, and this plasma is blown out by the gas flow in the channel due to the pressure gradient between the inside and the outside of the source. The "anode plasma" does not form when there is no blocking or covering such a large anode.